The realization of advanced lithographic technology beyond the 14 nm node requires the implementation of patterning materials and processes with ultimate performance in order to cope with intrinsically contrast-limited exposure tools. The reduction in imaging wavelength from the long-standing 193 nm ArF Lithography (DUV) to the expected 13.5 nm Extreme Ultraviolet Lithography (EUV) will improve the resolving power of ultimate optical systems used by the semiconductor industry, and is expected to enable the extension of Moore's Law to the 10 nm node and beyond.
The optical resolving power of ArF and EUV scanners can only be realized as a material pattern formed on a semiconductor substrate if the spatial resolution of the imaging material is commensurate to the quality of the delivered aerial image. At the same time, the imaging layer is required to carry acceptable sensitivity to the imaging wavelength and provide low line-width roughness (LWR), good adhesion to the underlying substrate and large post apply bake (PAB) and post exposure bake (PEB) latitude. Chemically amplified resists have become the workhorse of the lithographic industry due to the ability to satisfy the aforementioned requirements in a sustainable manner across multiple technology nodes.
Utilizing chemically-amplified (CA) resists is typically considered within the context of positive-tone resist imaging and development, where the exposed regions of a photoresist are removed by an aqueous basic developer (tetraalkylammonium hydroxide (TMAH)) after a post-exposure bake (PEB) step, while the unexposed regions remain insoluble. This is referred to as positive-tone development (PTD).
One way to reverse the tone of a chemically amplified resist originally designed to be developed in the positive tone with aqueous base developer is to utilize an organic developer that removes the unexposed portion of the resist film, while the exposed regions remain unaffected. This process is known as negative-tone development (NTD) and has found extensive applications in the area of 193 nm double patterning using bright field masks, particularly in the case of small feature openings such as contact hole and trench patterning. Hereinafter, NTD will be used to refer exclusively to refer to the use of organic solvents as developers to produce negative-tone images.
The resist contrast of the NTD process is determined by the solubility differences between the relatively nonpolar unexposed resist and the more polar resist material that is generated in the exposed regions of the film. As mentioned previously, in the NTD process the organic solvent dissolves the unexposed areas, and creates a negative image of the exposed chemically amplified photoresist. Processing of a chemically amplified resist in a NTD fashion utilizing anisole as the organic solvent developer was first reported by J. G. Maltabes, S. J. Holmes, J. R. Morrow, R. L. Barr, M. Hakey, G. Reynolds, W. R. Brunsvold, C. G. Wilison, N. Clecak, S. MacDonald, and H. Ito, 1× Deep UV Lithography With Chemical Amplification for 1-Micron DRAM Production, SPIE Vol. 1262, Advances in Resist Technology and Processing VII (1990), pp. 2-7.
There are a number of limitations, however, with the industrial application of NTD. Due to toxicological, environmental, and especially flammability issues there are a limited number of organic solvents that are compatible with semiconductor manufacturing. This is a serious impediment to finding the optimum developing solvent for a given resist. At this time it appears there are less than 6 organic solvents that are usable on fabrication development tracks.
Current high performance resists are highly optimized for development in aqueous TMAH developer. Many positive-tone resists perform poorly in negative tone development. Some don't work at all, and many functional resists exhibit significant defect, profile and film thinning problems in the exposed areas. Radical changes in the photoresist chemistry to improve NTD performance in acceptable NTD developing solvents would require extensive work to match the positive-tone performance that has been optimized over the last three decades.
There exists a need in the industry for a generalized process to improve the NTD performance of a CA resist developed with organic solvent to provide a negative tone image. The detailed materials and process to accomplish this will be disclosed below.